Typically the production of sponge metal in moving bed vertical reduction reactors involves the reduction of pebble-sized particles or lumps of metal oxides, ores or the like which descend downwardly through a reduction zone counter to a suitable upwardly flowing stream of hot reducing gas, largely composed of carbon monoxide and hydrogen at temperatures on the order of 850.degree. C. to 1100.degree. C. and for iron oxide preferably 900.degree. C. to 1000.degree., and modifying or carburizing the reduced metal oxides in a lower zone of the reduction reactor and discharging and conveying the hot metallized product without further modification directly to electric arc furnaces or the like by means of any appropriate conveying means, or hot briquetting the discharged product by means of a briquetting machine directly coupled to the reduction reactor.
Sponge iron produced in an apparatus and by a process of this type is disclosed in Beggs et al. U.S. Pat. No. 4,188,022 and Beggs et al. divisional U.S. Pat. No. 4,251,267 wherein iron oxide pellets or other particulate iron ore feed are charged to a vertical shaft furnace having an upper portion defining a reduction zone wherein a moving burden of particles is formed, and through which a hot reducing gas flows upwardly in countercurrent relationship to the movement of the burden, and a bottom portion extending into a hopper, which latter forms a gas disengaging plenum through which a co-current downwardly flowing sealing hot inert gas is disengaged from the descending pellets. Since the seal gas pressure is sufficient to prevent downflow of reducing gas leakage into the lower portion of the furnace, the amount of reducing gas leakage is kept to a minimum. Thus the gas leakage from the furnace through the furnace discharge pipe is principally recirculated seal gas containing only small amounts of CO and H.sub.2 reducing gas. The seal gas is preferably a gas having a high nitrogen content, such as the product of combustion containing no free oxygen.
The principal problem to which that patent is directed is to prevent leakage to the atmosphere of highly combustible and toxic furnace gas from the furnace discharge outlet. The patent teaches that the mere use of valves or locks have been tried without success in hot discharge direct reduction furnaces, because of their tendency to become bound or stuck because of the softness and stickiness of the hot reduced iron product which usually causes such valves to leak. Although a hot briquetting machine is coupled to the furnace discharge, it is isolated from the highly flammable gases maintained at relatively high pressure in the reduction furnace by means of the inert seal gas system at the lower end of the furnace, which is intended to insure that the gases present at the furnace discharge are non-flammable despite the character of the gases present within the reduction furnace.
That patent relies on the use of a hot inert gas which has to be circulated downwardly through the burden to avoid the leakage of the reducing gases. Such method and apparatus has the disadvantage that the capital investment is increased because of the ancillary equipment needed to maintain a flow of hot inert gas.
See also U.S. Pat. Nos. 4,734,128 and 4,834,792 for two other references showing hot discharge of sponge iron from a moving bed direct reduction reactor (with addition of natural gas to either the discharge zone or to the reducing zone, for control of carburization). No co-current flows are taught relative to the descending moving bed.
Sanzenbacher in U.S. Pat. No. 4,536,213 teaches use of natural gas and/or process gas containing significant amounts of higher hydrocarbons as a source for the reducing gas in a similar process and apparatus for the production of sponge iron in a moving bed Vertical reduction reactor. The natural/process gas is exposed first to the hot descending burden, which acts as a catalyst to crack the higher hydrocarbons, prior to separating, cleaning, and passing such gas through the reformer. In the first illustrated embodiment, the gas moves through the reactor only upwardly in the reducing zone countercurrent to the descending hot burden. In the second illustrated embodiment, the natural/process gas flows downwardly in a co-current direction with the descending burden in an intermediate portion of the reactor, below the reducing zone. There is no disclosure that the natural/process gas is heated. Furthermore that gas is withdrawn and the burden is subsequently cooled by an upflow countercurrent of cooling gas to discharge the burden cold. The present invention avoids this cooling effect.
Hiseh in U.S. Pat. No. 4,160,663 and U.S. Pat. No. 4,212,452 discloses a method and apparatus for the solid fuel direct reduction of iron ore carried out in a moving bed vertical shaft furnace to which ore and a solid fuel (which may be coal, charcoal, or any cellulosic material) are fed. While this does show some co-current flow of hot gases, it does so without controlled hot discharge.
The shaft furnace has three zones, (A) a gasification and initial reduction zone at the uppermost portion, (B) a second or final reduction zone, and (C) a third cooling and carburizing zone. In the initial reduction zone (A), the fuel is gasified by a controlled introduction of oxygen, steam and regenerated top gas and the conditions are controlled to produce CO and H.sub.2 diluted with CO.sub.2 and steam, while preheating and initiating reduction of the iron ore. The ore, fuel, and gases descend co-currently into a second or final reduction zone (B), wherein hot hydrogen-enriched reducing gases are introduced in the middle of the (B) zone. Off gases are removed from the upper portion and from the bottom of the second zone (B), cooled, purified of dust and carbon dioxide and, if necessary, of sulfur. A portion of the purified gases is mixed with air and burned to produce hot steam and power and to extract oxygen from air. The remaining purified gases are divided into two portions, one portion is enriched with H.sub.2 and together with hot steam is introduced into the middle of the second zone (B). This flows both upwardly (countercurrent) and downwardly (co-current) through the descending burden. The other portion of the remaining purified gases is cooled and dehumidified and introduced into the third zone (C) near the bottom of the furnace above and below a grate for cooling and carburizing. The sponge iron from the second zone enters the third cooling and carburizing zone (C), where it descends counter-currently to the rising dehumidified gases which cool and carburize the sponge iron. Finally the sponge iron is discharged cold.
Celada et al in U.S. Pat. No. 3,816,102 shows a split flow of hot reducing gas in the reduction zone of a moving bed vertical shaft furnace for the production of sponge iron (where there is the usual counter-current flow in the upper part of the reducing zone and a co-current descending flow of reducing gas relative to the descending burden of ore in the lower portion of the reducing zone). However, this patent disclosed a cooling zone and teaches cold discharge of the burden.
Price-Falcon et al in U.S. Pat. No. 4,793,856 (the contents of which are incorporated herein by reference) teach a process for the production of sponge iron where the usual upward counter-current flow of hot reducing gas in the reduction zone of a moving bed vertical shaft reactor is supplemented by a downward co-current flow of cooling gas along the walls of the reactor (to reduce the tendency of the burden to agglomerate and stick to the walls, thus permitting the reduction reaction to proceed at a higher than normal overall temperature thus resulting in more efficient operation). However, the counter-current flow is not only a cooling flow, but the reactor has a cooling zone, and cold discharge of the burden is taught.
It is accordingly an object of the present invention to provide method and apparatus for the gaseous reduction of iron ore in moving bed vertical reduction reactors into a highly metallized sponge iron product which is hot discharged at an average bulk temperature maintained at a level suitable for immediate melting or hot briquetting by suitable prevention and/or compensation for heat loss during the passage of the bed along the discharge zone of the shaft furnace.
Other objects and advantages of the invention will in part become obvious and in part be particularly pointed out hereafter.